1. Field of the Invention
The present invention generally relates to position determination utilizing a Global Positioning System (GPS) and, more particularly, to the determination and marking of position in an underwater environment utilizing GPS information.
2. Description of the Prior Art
Terrestrial Global Positioning Systems (GPS) have been available for several years. Such systems utilize communications with a plurality of satellites which are in known locations (e.g. geostationary orbits) for location of position of a GPS receiver with high accuracy (e.g. usually about six feet or less) on or above the earth's surface. Unfortunately, GPS is not effective underwater due to the inability of GPS signals to properly propagate in water. Current state of the art in underwater use of GPS as a position locator is restricted to an array towed by a diver or underwater vehicle in which the actual GPS receiver remains on a surface float flexibly tethered to the diver or vehicle. Naturally, in this configuration, accuracy is restricted to the inherent tolerances of the GPS receiver combined with error introduced by the actual vertical proximity (and related horizontal offset from the GPS receiver) of the diver or vehicle which is obviously influenced by prevailing currents and the length of the tether. Although GPS can be deployed on the water in a rather cumbersome configuration, such as described above, current conventional thought in this area states that GPS is not practical to use underwater.
However, for some purposes such as underwater salvage or archaeology in the hostile environment of the ocean or other bodies of water, especially when subject to changes of visibility conditions, accurate position location can be even more critical than some other GPS applications. For example, when it is necessary to leave a diving site of interest, it may be difficult to relocate the same site at a later time under different conditions, particularly where the sub-surface terrain may have been altered by intervening current or weather conditions. In the field of archaeology, it is also often important to make measurements and determine directions to accurately document the locations of artifacts as discovered. Further, it is considered to be an enhancement to safety of personnel or equipment to accurately track the position of divers or submersible devices while submerged particularly when it is inconvenient or undesirable to maintain a tether to the submerged personnel or equipment.
A system exists and is commercially available for informing divers, through sonar signaling from a tethered transmitter which may be affixed, for example, to a boat anchor, which will provide a distance and bearing from the diver to the sonar transmitter. This system is known and sold as the “EyeSee Sonar”™ by Xios Diving Technology of Couvet, Switzerland. The EyeSee™ system employs a sonar transmitter of small size which can be tethered to a support boat in a manner not critical to its use to transmit a sonar signal which has good transmissiveness through water as a compressional wave. A receiver unit is worn by the diver and contains a processor for computing a range and bearing from a receiver carried or worn by the diver to the transmitter and displaying the range and bearing to the diver. It is claimed by the manufacturer that the operational range of the EyeSee™ system is at least 3,300 feet. However, the distance indicated by the EyeSee™ system is a direct distance (e.g. effectively line-of-sight) between the transmitter and receiver and does not reflect the difference in horizontal location between the transmitter and receiver unless the diver and transmitter are at the same depth. Further, the EyeSee™ system has no capability of interfacing with a GPS system in any way or determining or reporting geographically or globally referenced data but only the position of the receiver relative to the transmitter by means of a sonar signal. Further, no provision is made in the EyeSee™ system for correction of the distance or bearing determination for effects such as relative salinity, temperature and other conditions of water which can affect the propagation speed of a sonar signal by which the distance determination is made. Furthermore, there is no provision in the EyeSee™ system for the communication of any data between the transmitter and receiver with or superimposed upon the sonar signal and the transmitter of the EyeSee™ system functions merely as a beacon while the receiver displays only distance and bearing to the transmitter for the limited purpose of allowing the diver to more readily locate the transmitter and, from that location, locate the support boat by following the transmitter tether, anchor rope or the like.